Fallen, are you okay with a patch replacing the textbox with a description with a text child node in chooseCalendarDialog.xul? This works for me in a test:
<description id="prompt"></description>
and
document.getElementById("prompt").textContent = window.arguments[0].promptText;

Looks like the review comment from bug 323085 comment 17 was never actually taken care of ;-)
Yes, that sounds good. While you are here, could you also add some accessibility features, I think it was:
<description id="prompt" for="id-of-listbox"/>
Please doublecheck.